1. Field of the Invention
The present invention relates to a surface acoustic wave device having a balanced-to-unbalanced conversion function and a filtering function, and a communication device including such a surface acoustic wave device.
2. Description of the Related Art
In recent years, the size and weight of communication devices, such as portable telephones, have been substantially reduced. Accordingly, not only has the number and size of components been reduced, but also multifunctional components have been developed.
Under such conditions, surface acoustic wave devices used in the RF stage of portable telephones, which have a balanced-to-unbalanced conversion function or the function of a so-called balun, have been rigorously studied in recent years and have commonly been used in GSM, and the possibility of these devices being used in PCS, DCS, etc., in future is very high.
Such surface acoustic wave devices having a balanced-to-unbalanced conversion function are disclosed in Japanese Unexamined Patent Application Publication No. 6-204781 and Japanese Unexamined Patent Application Publication No. 11-145772. In the above-mentioned surface acoustic wave devices, for example, the structure shown in FIG. 4 is widely used. The surface acoustic wave device shown in FIG. 4 includes a surface acoustic wave filter 3 shown in FIG. 5 and a surface acoustic wave filter 4 shown in FIG. 6, in which the phase is 180 degrees offset from that of the above surface acoustic wave filter 3, on a piezoelectric substrate (the piezoelectric substrate is omitted).
The surface acoustic wave filter 3 is a longitudinally coupled resonator type surface acoustic wave filter having three comb-shaped electrode portions (hereinafter called IDTs) where an IDT 31 and an IDT 33 are arranged on the left and right sides of a middle IDT 32 along the propagation direction of a surface acoustic wave, and reflectors 14 and 15 are arranged so as to sandwich the IDTs 31, 32, and 33.
The surface acoustic wave filter 4 is a longitudinally coupled resonator type surface acoustic wave filter having three IDTs where an IDT 41 and an IDT 43 are arranged on the left and right sides of a middle IDT 42 and reflectors 14 and 15 are arranged so as to sandwich the IDTs 41, 42, and 43.
In the surface acoustic wave filter 3 and the surface acoustic wave filter 4, the phase of the middle IDT 32 and IDT 42 is 180 degrees offset from each other, that is, the polarity of the electrode-finger portions 32a and 32b of the IDT 32 is opposite to the polarity of the electrode-finger portions 42a and 42b of the IDT 42.
The terminal connected to the electrode-finger portion 31b of the IDT 31 and the electrode-finger portion 33b of the IDT 33 defines one balanced terminal 6. The terminal connected to the electrode-finger portion 41b of the IDT 41 and the electrode-finger portion 43b of the IDT 43 defines another balanced terminal 7. The terminal connected to the electrode-finger portion 32b of the IDT 32 and the electrode-finger portion 42a of the IDT 42 defines a further unbalanced terminal 5.
In a surface acoustic wave device having a balanced-to-unbalanced conversion function, with respect to the transmission characteristics inside the passband between the unbalanced signal terminal and each of the balanced signal terminals, the amplitude characteristics must be equal and the phase must be 180 degrees offset between them and, outside the passband, the amplitude characteristics and the phase characteristics must be equal between them.
Regarding the amplitude balancing and phase balancing, when a surface acoustic wave device having the above-described balanced-to-unbalanced conversion function is referred to as a three-port device and, for example, the unbalanced input terminal is referred to as a first port and the balanced output terminals are referred to as a second port and a third port, respectively, the amplitude balancing and the phase balancing are defined as follows. Amplitude balancing=|A|, A=|20 log(S21)|−|20 log(S31)|, and phase balancing=|B−180|, B=|<S21−<S31|. Moreover, S21 is a transfer factor from the first port to the second port and S31 is a transfer factor from the first port to the third port. Regarding such balancing between balanced signal terminals, ideally the amplitude balancing is 0 dB and the phase balancing is zero degree inside the passband of the surface acoustic wave device.
However, in the surface acoustic device shown in FIG. 4, there are differences from the ideal balancing and the level of the differences cause problems when practically used. That is, in the surface acoustic wave filter 3 of the related surface acoustic wave device, each of the electrode-finger portions 31a, 32a, and 33a to which the outermost finger electrodes at the boundaries between the three IDTs (IDT31, IDT32, and IDT33) belong is grounded.
On the other hand, in the surface acoustic wave filter 4 of the related surface acoustic device, regarding the electrode-finger portions 41a, 42a, and 43a to which the outermost finger electrodes at the boundaries between the three IDTs (IDT41, IDT42, and IDT43) belong, the electrode-finger portions 42a of the IDT 42 is connected to the unbalanced terminal 5 to define a signal terminal and each of the electrode-finger portions 41a and 43a of the IDT 41 and IDT 43 is grounded.
Therefore, at the boundaries between the IDTs 31, 32, and 33 of the surface acoustic wave filter 3, since the grounded electrode-finger portions 31a, 32a, and 33a are adjacent to each other, no surface acoustic wave is excited. However, at the boundaries between the IDTs 41, 42, and 43 of the surface acoustic wave filter 4, since the electrode-finger portion 42a connected to the unbalanced terminal 5 produces a potential difference to the grounded electrode-finger portions 41a and 43a, a surface acoustic wave is excited.
In this manner, between the surface acoustic wave filter 3 and the surface acoustic wave filter 4, since there is a difference whether a surface acoustic wave is excited at the boundaries between adjacent IDTs or not, filtering characteristics are substantially different. This fact causes a problem in that the balancing, particularly the balancing inside the passband, is greatly deteriorated in the above-described related surface acoustic wave device.